1. Field of the Invention
The present invention relates to an optical film, a polarizing plate using the optical film, and an image display device using the optical or polarizing plate on the outermost surface of the display.
2. Description of the Related Art
In an image display device such as cathode ray tube display (CRT), plasma display panel (PDP), electroluminescent display (ELD) and liquid crystal display (LCD), an antireflection film is generally disposed on the outermost surface of the display so as to reduce the reflectance by utilizing the principle of optical interference and thereby prevent the reduction in contrast due to reflection of outside light or prevent the disturbing reflection of an image.
Such an antireflection film can be generally produced by forming, on a support, a low refractive index layer having a refractive index lower than that of the support to an appropriate film thickness. In order to realize a low reflectance, a material having a refractive index as low as possible is preferably used for the low refractive index layer.
Also, the antireflection film is used on the outermost surface of a display and therefore, requires high scratch resistance. In order to realize high scratch resistance in a thin film with a thickness of around 100 nm, strength of the film itself and firm adhesion to the underlying layer are necessary. For increasing the film strength, it is known to add an inorganic fine particle into the film.
As for the inorganic fine particle, in addition to the increase of the film strength, an attempt to reduce the refractive index by adding a hollow fine particle is being made. At this time, addition of an inorganic fine particle in a sufficiently large amount within the range not causing reduction in the film strength due to insufficient binder is important.
On the other hand, for reducing the refractive index of a material, a method of introducing a fluorine atom is known, and it has been proposed to use a fluorine-containing crosslinking material (see, JP-A-2003-222702 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)).
A technique of adding an inorganic fine particle to a fluorine-containing crosslinking material for forming a film having a low refractive index and excellent scratch resistance is also disclosed (JP-T-2005-535934 (the term “JP-T” as used herein means a “published Japanese translation of a PCT patent application”)). However, in this case, dispersibility of the fine particle is bad and when the fine particle is added in a large amount, the coated film may be whitened, which is in need of improvement.
As regards this improvement, addition of a dispersant for the inorganic fine particle may be considered, but a normal dispersant tends to increase the refractive index and decrease the strength of the coated film. Therefore, a technique of improving dispersibility of the fine particle and suppressing whitening of the coated film without decreasing the film strength and increasing the refractive index is being expected.
Also, a fluorine-based compound which can be usefully used as a lubricant, a surfactant or a water-repelling oil-repelling material has been proposed (JP-A-2006-045159). However, this compound is a special compound using a fluorine-based solvent and cannot necessarily satisfy the scratch resistance, and an improvement is being demanded.